A crashing issue

The impact of air bag warning light checks

Published:  15 July, 2014

Unfortunately, for every new rule there seems to be at least two ways around it, which are only now coming to light. People are passing up the opportunity to earn money from a genuine repair of a safety critical system. Yet it really doesn't have to be too hard to get a system fully operational again.

I recently had a Mercedes in for an SRS (Safety Restraints System) warning light being displayed and a subsequent MOT fail. The test centre was unable to help any further, so it arrived at my garage for the repair. The trouble codes stored in the memory were all related to the driver's side front crash sensor, communication error, short to plus/earth and interestingly, wrong sensor installed. It's fair to say that most people would take a leap of faith here and replace the driver's side front sensor but there was slightly more to it than first met the eye.

Time for the scope

The great advantage of dealing with crash sensors is that they normally come in pairs. Initially, I decided to get the oscilloscope out and have a look at the signals being generated by both front crash sensors, shown below.

Figure1                                                                        Figure2

If you are like me and don't have an encyclopaedia of trace patterns in your head or a previous example saved on file, then it's a good idea to make a comparison with a known good example. These crash sensors were accelerometers and belong to the umbrella term of 'smart sensors,' this just means the sensor can produce a signal independent of the sensed voltage which the controller can interpret.

In this example, the passenger side sensor (Figure 1) is producing a normal response and the driver's side (Figure 2) is reporting a fault. This is a good example of why to make a comparison because the error message could be misdiagnosed as a valid signal although it is reporting a fault.

This brings me back to the original set of trouble codes and the anomaly of 'wrong sensor installed' which in this case is the key to all of the problems. Accelerometer crash sensors are very sensitive to direction and orientation and typically only measure along one axis. The cause of this fault was the driver's side sensor which was actually installed upside down and the plastic locating feature on the sensor, which should prevent this from happening, was broken off, see below.

This in itself would not cause the short circuit fault but unfortunately it did mean that the multi-pin connection was pointing straight up and was therefore unprotected from the elements, resulting in corrosion, see below. We now had a complete picture of what had happened and an explanation for all the trouble codes. All that was left to do was to replace the sensor and reset the system but there was one more curve ball. Mercedes had modified this sensor and recommend replacing them as a pair (this seems to be becoming increasingly common). Always worth checking with your parts supplier to save yourself unnecessary aggravation!

One more thing...

There are pit falls to working with airbag systems, apart from the obvious safety concerns with explosive devices. Manufacturers frequently update components and require multiple upgrades when replacing parts. Control units and sensors with accelerometers must be installed correctly to prevent unusual fault messages. It is also one system where they often refer to sensor position by stating driver's or passenger side, so make sure you're not duped in to a wild goose chase when your diagnostic tool thinks you're looking at a left hand drive car - I learnt this the hard way with a Chrysler Voyager. Whether we like it or not, air bag system repairs are a necessary part of our industry, so hopefully we embrace it and earn money for effectively resolving faults.

By 2014 Top Technician winner Mike Harding

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